Mathematica competitions in Croatia

Let's divide the given table into $25$ smaller squares $2\times 2$. In each of these squares there is at most one even number and at most one number divisible by $3$. Hence, at most $50$ numbers in the table are divisible by $2$ or $3$. At least $50$ numbers remain, and each of them is equal to $1$, $5$ or $7$. By the box principle, at least one of these numbers appears at least $17$ times.